The present invention relates to a novel insertion device or, in particular, to a wiggler which is inserted to the linear part of an electron accelerator or an electron storage ring to emit a synchrotron radiation of high brilliance.
An insertion device consists of two oppositely facing arrays with a gap space therebetween each formed of permanent magnet blocks or consisting of two arrays each formed of permanent magnet blocks and blocks of a magnetically soft ferromagnetic material, such as iron or an alloy of iron and cobalt, in combination. FIG. 6A of the accompanying drawing illustrates a schematic perspective view of an insertion device consisting of two arrays of permanent magnet blocks alone, in which the small arrow appearing on the side surface of each of the magnet blocks indicates the direction of magnetization of the block. This insertion device is inserted into the straightly linear part of an electron accelerator or an electron storage ring in such a fashion as to sandwich a vacuum chamber between the magnet arrays as shown in FIG. 6A generating a sine-curved periodical magnetic field within the vacuum space between the magnet arrays as is shown in FIG. 6B. When electrons at a near-light velocity circulating in an electron accelerator are introduced into such a wiggled or undulated magnetic field along the z-direction of the magnet arrays, the electron beam causes meandering through the space in the direction indicated by the arrow e in FIG. 6A to emit a synchrotron radiation R from each meandering point as is schematically shown in FIG. 6C.
As is mentioned above, the insertion devices to generate the sine-curved periodical magnetic field can be classified into the Halbach type ones consisting of permanent magnet blocks alone and the hybrid type ones consisting of permanent magnet blocks and blocks of a magnetically soft ferromagnetic material in combination as disclosed in Nuclear Instruments and Methods, volume 288 (1983), pages 117-125 and Review of Scientific Instruments, volume 58(3), March, 1987. FIG. 7 is a schematic side view, as viewed in the direction of the x-axis, of the magnet block arrays in a Halbach type insertion device consisting of the permanent magnet blocks 41, 42 alone, each of which is magnetized in the direction indicated by the respective small arrow on the side surface of the block 41 or 42. The period of the sine-curved periodical magnetic field corresponds to the length P formed with four adjacent permanent magnet blocks.
In the hybrid type insertion device, as is illustrated in FIG. 8A by a schematic plan view as viewed in the direction of the y-axis, each of the magnet array consists of an alternate arrangement of permanent magnet blocks 41 and pole pieces 43 of a magnetically soft ferromagnetic material which serve to converge the magnetic fluxes. A period P of the sine-curved periodical magnetic field in this case has a length formed from two permanent magnets 41 and two pole pieces 43. The strength and distribution of the magnetic field accomplished in the above mentioned two types of insertion devices are substantially identical without particular differences in the performance excepting for an economical advantage in the hybrid type ones because the overall amount of the permanent magnets can be saved as compared with the Halbach type ones.
Insertion devices of these types can also be classified into undulators and wigglers depending on the value of the parameter K which is a function of the length of the period P and the strength of the magnetic field. Namely, an insertion device is an undulator or a wiggler when the value of K is about 1 or smaller or when the value of K is substantially larger than 1, respectively.
The present invention relates to a hybrid-type insertion device or, more particularly, to a hybrid-type wiggler. In a hybrid-type wiggler, as is illustrated by the schematic plan and side views in FIGS. 8A and 8B, respectively, each of the pole pieces 43 in the magnet array is sandwiched between two permanent magnet blocks 41 magnetized each in a reversed direction to that of the nearest magnet block along the direction of the array or center axis C and the magnetic flux is converged to the respective pole pieces 43 so that a strong magnetic field is generated in the gap space having a distance d between two arrays of the permanent magnets 41 and the pole pieces 43. As is shown in FIG. 8A, each of the pole pieces 43 has a dimension in the direction of the x-axis smaller than that of the permanent magnet blocks 41 in the same direction in order to facilitate conversion of the magnetic fluxes onto the center axis C along which the electrons travel.
Since wigglers are used for generating radiation of a particularly high energy or hard X-rays, the magnetic field generated in the gap space between the magnet block arrays must be strong enough. While the magnetic field can be increased by decreasing the distance d between the magnet block arrays, it is not practical to decrease the distance d of the gap space to be substantially smaller than 10 mm in order to ensure keeping of a space for the vacuum chamber. Although the magnetic field can be increased to some extent by using permanent magnet blocks of an increased volume, this means does not provide a solution of the problem because, in the hybrid-type wigglers, the magnetic field is limited by the magnetic saturation of the pole pieces 43 as the volume of the permanent magnet blocks 41 is increased and, in the Halbach-type wigglers, contribution to the magnetic field can be exhibited only by the volume portions of the permanent magnet blocks in the proximity to the center axis C and the volume portions remote from the center axis C have little contribution.
It is an estimation that the wiggler in a medium-size synchrotron radiating instrument is required to generate a magnetic field of at least 2 T as the peak value of the periodical magnetic field if hard X-rays are to be utilized in the instrument. Needless to say, the utilizability of any synchrotron radiation instruments can be increased as the magnetic field generated in the wiggler thereof is increased since synchrotron radiations of a wider energy range can be provided.
The present invention accordingly has an object to provide a hybrid-type wiggler capable of generating a high periodical magnetic field which cannot be obtained in the wigglers of the prior art.
Thus, the present invention provides a wiggler of the hybrid type consisting of a pair of oppositely facing arrays with a gap space therebetween each formed of a plurality of main permanent magnet blocks and a plurality of blocks of a magnetically soft ferromagnetic material, such as iron or an iron-cobalt alloy, as pole pieces alternately arranged in the longitudinal direction of the array, each main permanent magnet block in one array facing one of the main permanent magnet blocks in the other array and each pole piece in one array facing one of the pole pieces in the other array, in which each of the pole pieces is sandwiched at the lateral surfaces with a pair of auxiliary permanent magnet blocks.